Aims of this investigation are 1) to clarify the pattern of follicle development in fetal and neonatal mammalian ovaries, and the source of the follicle cells, and 2) to establish whether or not folliculogenesis continues in the ovaries of adult mammals other than Prosimians. The study will be performed on ovaries of mice and rhesus macaques (Macaca mulatta); fetal and immature mouse ovaries at various stages of development will be utilized for the study of patterns of follicle morphogenesis during fetal and neonatal periods, while adult rhesus macaque ovaries will represent the material to study the possible occurrence of folliculogenesis during sexual maturity. The investigation will be conducted by means of high resolution light microscopy and electron microscopy on serially sectioned tissue.